(495c) Stress Evolution and Cracking in Drying Polymer Films

Thin coatings of polymer films find numerous industrial applications in the formation of protective and functional coatings. These coatings are initially wet, and then evaporation of the solvent leads to the solidification of the film. This solidification process is often accompanied by shrinkage of the film. The strong adhesion between the film and substrate prevents the contraction of the film in the transverse direction, whereas it can freely shrink in the normal direction. This constraint shrinkage in the plane of coating develops transverse stresses in the film. If these stresses exceed a critical value, the polymer film may fracture, thereby compromising the integrity of the film. On the other hand, a weak adhesion between the film and substrate leads to the debonding of the film instead of cracking. The occurrence of cracks during drying of polymer films makes the drying process very complex, and it is crucial to understand this mechanism to get the crack-free films.

In the present work, thin films of a polymer solution were cast on substrates of varying stiffness and investigated for cracking as a function of film thickness and substrate modulus. We found that the stiffness of the substrate influence cracking. Decreasing the stiffness of the substrate increases the extent of cracking. Transverse stress was measured during the drying of the film using glass as a cantilever substrate and related to the crack density (Figure 1). The residual stress was found to be independent of dry coating thickness for a low thickness range (≤ 20 µm) despite a few cracks that appeared in the film above a critical thickness. Micro-Raman spectroscopy confirms the gradient in polymer concentration (skin formation) across the thickness during drying of films having large thicknesses. The skin formation and extensive cracks followed by delamination from the substrate led to a lower value of residual stress for very large thicknesses films (≥40 µm). We compare the measurements with predictions of a model that accounts for the elastic energy released due to cracking.